In recent years, small electronic devices, represented by mobile terminals, have been widely used and urgently required to reduce the size and weight and to increase the life. Such requirement has advanced the development of particularly small, lightweight secondary batteries with higher energy density. These secondary batteries are considered to find application not only for small electronic devices but for large electronic devices such as, typically, automobiles as well as power storage systems such as, typically, houses.
Among those, lithium-ion secondary batteries are easy to reduce the size and increase the capacity and have higher energy density than those of lead or nickel-cadmium batteries, receiving considerable attention.
The lithium-ion secondary battery has positive and negative electrodes, a separator, and an electrolyte. The negative electrode includes a negative electrode active material related to charging and discharging reactions.
A negative electrode active material, which is usually made of a carbon material, is required to further improve the battery capacity for recent market requirement. Use of silicon as a negative electrode active material is considered to improve the battery capacity, for silicon has a logical capacity (4199 mAh/g) ten times larger than does graphite (372 mAh/g). Such a material is thus expected to significantly improve the battery capacity. The development of silicon materials for use as negative electrode active materials includes not only silicon as a simple but also alloy thereof and a compound thereof such as typically oxides. The consideration of active material shapes for carbon materials ranges from a standard application type to an integrated type in which the materials are directly accumulated on a current collector.
Use of silicon as a main material of a negative electrode active material, however, expands or shrinks a negative electrode active material when charging or discharging, thereby making the negative electrode active material easy to break particularly near its surface layer. In addition, this active material produces ionic substances in its interior and is thus easy to break. The breakage of the surface layer of the negative electrode active material creates a new surface, increasing a reaction area of the active material. The new surface then causes the decomposition reaction of an electrolyte and is coated with a decomposition product of the electrolyte, thereby consuming the electrolyte. This makes the cycle performance easy to reduce.
Various materials and configurations of a negative electrode for a lithium-ion secondary battery mainly using a silicon material have been considered to improve the initial efficiency and the cycle performance of the battery.
More specifically, a vapor deposition method is used to accumulate silicon and amorphous silicon dioxide simultaneously so that better cycle performance and greater safety are achieved (See Patent Document 1, for example). Moreover, a carbon material (an electronic conduction material) is disposed on the surface of silicon oxide particles so that a higher battery capacity and greater safety are achieved (See Patent Document 2, for example). Moreover, an active material including silicon and oxygen is produced to form an active material layer having a higher ratio of oxygen near a current collector so that improved cycle performance and higher input-output performance are achieved (See Patent Document 3, for example). Moreover, silicon active material is formed so as to contain oxygen with an average content of 40 at % or less and with a higher oxygen content near a current collector so that improved cycle performance is achieved (See Patent Document 4, for example).
Moreover, a nano-complex including-Si-phase, SiO2, MyO metal oxide is used to improve the first charge and discharge efficiency (See Patent Document 5, for example). To improve the cycle performance, SiOx (0.8≤x≤1.5, the range of particle size=1 μm to 50 μm) and a carbon material are mixed and calcined at a high temperature (See Patent Document 6, for example). The active material is controlled by adjusting a mole ratio of oxygen to silicon in a negative electrode active material in the range from 0.1 to 1.2 so as to hold a difference between the maximum and the minimum of the oxygen-to-silicon mole ratio near the interface between the active material and a current collector at 0.4 or less, so that improved cycle performance is achieved (See Patent Document 7, for example). Moreover, a metal oxide containing lithium is used to improve the battery load characteristic (See Patent Document 8, for example). To improve the cycle performance, a hydrophobic layer such as a silane compound layer is formed on the surface of a silicon material (See Patent Document 9, for example).
Moreover, a silicon oxide is used and coated with graphite coating to give electric conductivity so that improved cycle performance is achieved (See Patent Document 10, for example). In Patent Document 10, the graphite coating exhibits a Raman spectrum that has broad peaks at shift values of 1330 cm−1 and 1580 cm−1, and their intensity ratio I1330/I1580 satisfies 1.5<I1330/I1580<3. Similarly, a particle having a silicon microcrystal phase dispersed in a silicon dioxide is used to achieve a higher battery capacity and improved cycle performance (See Patent Document 11, for example). Moreover, a silicon oxide having a silicon-to-oxygen atomicity ratio of 1:y (0<y<2) is used to improve overcharge and overdischarge performance (See Patent Document 12, for example). Furthermore, a mixed electrode of silicon and carbon is produced with the silicon ratio set to 5 wt % or more and 13 wt % or less to achieve a higher battery capacity and improved cycle performance (See Patent Document 13, for example).